The present invention relates to the novel compound, acid 1,4,7,10-tetraazacyclododecane-1,4-diacetic of formula (I), its complexes with paramagnetic metal ions and physiologically compatible salts thereof, as well as to the preparation thereof and the use thereof for the preparation of chelating agents. 
The compound of formula (I) is a novel chelating agent for bi-trivalent metal ions and is also an important intermediate for the synthesis of 1,4,7,10-tetraazacyclododecane derivatives chelating agents, functionalized at the 1-and 4-positions with the acetic residue.
The compound of formula (I) is the starting material for the synthesis of multidentate derivatives which are capable of complexing different metals, some of which have applications in the biomedical field, such as gadolinium complexes of said derivatives, which are used in diagnostic as contrast agents for the magnetic resonance technique (Magnetic Resonance Imaging, MRI).
Such complexes have been described, inter alia, in EP 325762.
Therefore, the object of the present invention is the compound of formula (I): 
as well as its chelated complex salts with bi-trivalent metal ions having atomic number from 20 to 31, 39, 42, 43, 44, 49, or from 57 to 83, as well as their salts with anions of physiologically acceptable organic acids, selected from, for example, acetate, succinate, citrate, fumarate, maleate, oxalate, or with anions of inorganic acids such as halo acids ions, specificallychlorides, bromides, iodides.
Metal ions suitable for preparing chelated complex salts with the novel chelating agent (I) are mainly bivalent or trivalent ions of the elements having atomic number variable from 20 to 31, 39, 42, 43, 44, 49, or from 57 to 83; particularly preferred being Fe(2+), Fe(3+), Cu(2+), Cr(3+), Gd(3+), Eu(3+), Dy(3+), La(3+), Yb(3+) or Mn(2+) also radioisotopes such as 51Cr, 67Ga, 68Ga, 111In, 99mTc, 140La, 175Yb, 153Sm, 166Ho, 90Y, 149Pm, 177Lu, 47Sc, 142Pr, 159Gd, 212Bi.
The novel compounds of the present invention have a good tolerability; moreover, their water solubility and the limited osmolality of their solutions are a further advantageous characteristic which makes them particularly suitable for use in nuclear magnetic resonance.
Both soluble and less soluble compounds are useful for the oral and enteral administrations and, therefore, for the imaging of the gastronitestinal (GI) tract.
As far as the parenteral administration is concerned, the compounds are preferably formulated as a sterile aqueous solution or suspension, whose pH can range for instance from 6.0 to 8.5.
These aqueous solutions or suspensions can be administered in concentrations ranging from 0.002 to 1.0 mol.
These formulations can be lyophilized and supplied as they are for reconstitution before use.
For the GI use or for the injection in the body cavities, such agents can be formulated as a solution or suspension containing suitable additives which can control viscosity.
In the oral administration they can be formulated according to preparation methods commonly used in the pharmaceutical practice, possibly also as coated formulations, in order to get additional protection from the stomach acid pH, by preventing the release of the chelated metal ion occurring in particular at pH which are typical of gastric juices.
Other excipients, for instance sweeteners and/or flavouring agents, can also be added according to known techniques of pharmaceutical formulation.
In the diagnostic field, the chelated complex salts of this invention can be used as contrast agents, while as radiopharmaceuticals in nuclear medicine, they are useful both in the diagnostic and therapeutic sector.
In this case, however, the metal ion which is chelated is a radioisotope, for instance 51Cr, 67Ga, 68Ga, 111In, 99mTc, 140La, 175Yb, 153Sm, 166Ho, 90Y, 149Pm, 177Lu, 47Sc, 142Pr, 159Gd and 212Bi.
The compounds object of the present invention can optionally be chemically conjugated with suitable macromolecules or included in suitable carriers.
Preferred anions of inorganic acids suitable for salifying the chelated complex salts of the invention comprise, in particular, halo acids ions such as chlorides, bromides, iodides or other ions, such as sulfate.
Preferred anions of organic acids suitable for the above aim comprise those of the acids conventionally used in pharmaceutical technique for the salification of basic substances, such as acetate, succinate, citrate, fumarate, maleate.
Preferred amino acid anions comprise, for example, those of taurine, glycine, lysine, arginine or ornithine, or of aspartic and glutamic acids.
The chelated complex salts of compound (I) with the metal ions defined above are prepared according to procedures known in literature, by reacting compound (I) with the oxide or the halide of the selected metal ion.
More specifically, the reaction is carried out in water or in a suitable water-alcohol mixture, and the temperature can range from 25xc2x0 C. to 100xc2x0 C., preferably from 40xc2x0 C. to 80xc2x0 C.
The choice of the metal ion and of any neutralizing ion is closely related to the intended use of the complex to be prepared.
The preparation of the novel compound, manganese chelated complex of 1,4,7,10-tetraazacyclododecane-1,4-diacetic acid, which is in the neutral form and therefore does not require the formation of a physiologically compatible salt, is described in the Experimental section. 
The compound of formula (I) was surprisingly prepared starting from octahydro-2a,4a,6a,8a-tetraazapentalen[1,6-cd]pentalene (CAS RN 54364-78-2) of formula (II), and from 1,2,3,4,6,7,8,9-octahydro-5H-4a,7,9a-triaza-2a-azoniacycloocta[cd] pentalene chloride of formula (IV), both obtainable according to known methods from 1,4,7,10-tetraazacyclododecane (III) (commonly named Cyclen), according to the following Scheme 1, through the formation of the novel compound of formula (V), 1,4,7,10-tetraazabicyclo[8.2.1 ]tridecane-13-on-4,7-diacetic acid, which is also an object of the invention: 
The compound of formula (II) quickly and reversibly dissociates in water to give the compound of formula (IV), the respective preparations of said compounds being already disclosed in U.S. Pat. No. 3,932,451 and in a paper (Richman et Simmons, Tetrahedron, 30, 1769, 1974), for the use of both of them in photography.
The compound of formula (II) is obtainable by benzene extraction from the alkalinized solution of compound (IV), which can easily be prepared in quantitative yields from 1,4,7,10-tetraazacyclododecane (commercial product, commonly named Cyclen) of formula (III), by reaction with ethyl orthocarbonate and an equivalent of hydrochloric acid in ethanol, as shown in the following Scheme: 
The compound of formula (IV) is in prototropic equilibrium at room temperature with compound (II) and with the dicationic compound (VI). This equilibrium is also of conformational type, thus causing the magnetic equivalence of protons of compound (II).
U.S. Pat. No. 3,932,451 discloses the preparation of these compounds starting from 2,3,5,6-tetraidro-1H-imidazo[1,2a]-imidazole, as shown in the following Scheme: 
The compound of formula (II) quickly reconverts to compound (VII) by reaction with ethyl chloroformate, thus giving further structural evidences.
From such a behaviour of the compounds of formula (II) and (IV), documented in literature, no uses of these compounds as useful intermediates for the synthesis of 1,4-disubstituted 1,4,7,10-tetraazacyclododecane derivatives could be expected.
It is a further object of the invention the process for the preparation of the novel compound of formula (I), starting from known compounds of formula (II) or (IV), through the formation of the novel compound of formula (V), comprising the following steps represented in Scheme 2: 
in which
step a) is the alkylation reaction in basic conditions with an acetic acid reactive derivative, XCH2COOH, in which X is a halogen;
step b) is the basic hydrolysis under pressure and at a temperature ranging from 150-220xc2x0 C.
The alkylation conditions in step a) are conventional ones: the reaction temperature can range from 30 to 70xc2x0 C.; the reaction time usually ranges from 10 to 25 hours; the basic pH ranges from 10 to 12 and is obtained by addition of a inorganic base, preferably sodium or potassium hydroxide; the amount of alkylating agent is stoichiometric or in a slight excess (up to 50%).
Preferred conditions in the presence of at least 2 mols of BrCH2COOH as alkylating agent per mol of starting product are the following: temperature 45xc2x0 C.; reaction time 21 hours; pH 11.5.
Compound of formula (V) is purified by elution of the acidified final solution on a polystyrene adsorbing resin, such as XAD-1600, to remove the salts, and subsequent recovery of the desired product.
Compound (V), whose structure was confirmed by spectroscopic analysis (1H-NMR, 13C-NMR, IR and MS), shows a high stability under hydrolytic conventional conditions.
It has surprisingly been found that the basic hydrolysis of this compound at high temperature and under pressure causes the loss of the carbonyl bridge, while keeping the acetic residues intact, thereby yielding simply and efficiently compound of formula (I).
The basic hydrolysis is carried out in aqueous medium at basic pH by addition of an amount of inorganic base, as defined above, corresponding to 4-7 mols per mol of compound (V) at temperatures of 150-220xc2x0 C.; pressure depending of course by the selected temperature to carry out the reaction according to ideal gas law; the reaction time ranging from 15 to 30 hours.
Preferred conditions are as follows: temperature 195xc2x0 C. and pressure 10 bars; 5 mols of NaOH per mol of compound (V); reaction time 22 hours.
The compound of formula (I) is in its turn a useful starting product for the synthesis of the compounds of general formula (VIII), 
which are useful chelating agents of paramagnetic metal ions, for the preparation of contrast agents for magnetic resonance imaging, as described in EP 325762.
It is therefore an object of the invention the process for the preparation of compounds (VIII), starting from compound (I), by alkylation, according to known methods, with an excess of alkylating agent Rxe2x80x94CH(X)xe2x80x94COY of formula (IX), optionally followed by hydrolysis of the ester groups present, as shown in the following Scheme 3: 
in which
R is a hydrogen atom, a straight or branched or cyclic C1-C6 alkyl group, unsubstituted or substituted by 1 to 10 oxygen atoms, or a C1-C20 alkyl group, optionally interrupted by a phenylene, phenylenoxy or phenylenedioxy group, in its turn substituted by a straight or branched C1-C6 alkyl group, unsubstituted or substituted by 1 to 7 hydroxy groups or 1 to 3 C1-C7 groups; the aromatic group can be unsubstituted or substituted by alkoxy groups or by halogens, carboxy, carbamoyl, alkoxycarbonyl, sulfamoyl, hydroxyalkyl, amino, acylamino, acyl, hydroxyacyl groups;
X is a halogen or a sulfonic acid reactive residue,
Y is a group xe2x80x94OH or xe2x80x94OR1, wherein R1 is a straight or branched C1-C4 alkyl group; when Y is xe2x80x94OR1, the ester groups are subjected to a hydrolysis step, according to known methods, to obtain compounds (VIII).
The alkylating agents of formula (IX) corresponding to compound of formula (X), Rxe2x80x94CH(X)xe2x80x94COOH, in which X is bromine or chlorine, are preferred; the alkylating agents of formula (XI), XCH2COOH, in which R is a hydrogen atom and X is bromine or chlorine, being most preferred.
In the other cases the alkylating agent of formula (IX) can be selected from compounds that are commercially available or the preparation of which has already been described in literature (see for example WO 93/24469 or EP 325762), or from those still to synthesize, using for example known methods for the preparation of suitable precursors (for example, in case of acyl chlorides xcex1-halogen derivatives, see: Harpp et al., J. Org. Chem., 40, 3420, 1975), and subsequent transformation into the desired product.
Preferably R can be selected from the group consisting of: H or a straight or branched alkyl group, such as a methyl, ethyl, propyl, isopropyl, butyl, isobutyl group, in its turn substituted by hydroxy groups or interrupted by oxygen atoms, as defined above.
When an aromatic group is present in R, particularly preferred are the phenyl, benzyl, phenylmethoxymethyl groups.
Particularly preferred are 3-(phenylmethoxy)propanoic acid reactive derivatives, such as 2-bromo-3-(phenylmethoxy)propanoic acid, the preparation of which is described in Grossman et al., Chem. Ber., 91, 538, 1958, and 2-chloro-3-(phenylmethoxy)propanoic acid (CAS RN 124628-32-6), prepared analogously to the brominated derivative.
On the other hand, the group R1 is preferably selected from: methyl, ethyl, isopropyl, butyl, tert-butyl.
The reactive group X can be selected, by way of example, from the group consisting of halogens (Cl, Br, I), or it is a mesylate (MeSO2Oxe2x88x92), benzenesulfonyloxy (PhSO2Oxe2x88x92), nitrobenzenesulfonyloxy (p-NO2PhSO2Oxe2x88x92), tosylate (TsOxe2x88x92) or triflate (CF3SO3xe2x88x92) group.
Particularly preferred are the compounds in which X is a halogen, a bromide or a chloride being most preferred.
The alkylation of compound (I), when Y is the hydroxy group, can conveniently be performed with secondary carboxylic acids reactive derivatives, such as 2-bromopropionic acid, in aqueous alkaline solution, at temperatures from 25 to 55xc2x0 C.
Particularly preferred are the alkylating agents of general formula (X), in which Y is a hydroxyl group, corresponding to bromoacetic acid (commercially available product), 2-bromopropionic acid (commercially available product), 2-bromobutyric acid (commercially available product).
On the other hand, when the alkylation reaction is carried out with an ester derivative of compound (IX), the reaction solvent can suitably be selected from dipolar aprotic solvents, in particular from dimethylformamide (DMF), dimethylacetamide (DMA), dimethylsulfoxide (DMSO), acetonitrile (CH3CN) and N-methylpyrrolidone, and the reaction is carried out in the presence of an organic base, preferably a tertiary aliphatic amine selected from triethylamine (TEA), diisopropylethylamine and tributylamine.
In this case it can be convenient to transform also the acid groups (xe2x80x94COOH) present in compound (I) into the ester groups (xe2x80x94COOR1), in order to promote the alkylation reaction, depending on the reactivity of the alkylating agent itself.
The reaction temperature will range, in this case, from 0 to 80xc2x0 C., depending on the reactivity of the selected alkylating agent.
In this case, the alkylation reaction will be followed by basic hydrolysis of the resulting diester, in conventional conditions, to obtain the desired compound of formula (VIII).
By way of example of the huge potentialities provided by this synthetic route, the synthesis of the novel compound, xcex1,xcex1xe2x80x2-bis(methyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid: 
as well as that of xcex1,xcex1xe2x80x2-bis((phenylmethoxy)methyl)-4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid: 
are reported in the Experimental section, in which catalytic hydrogenation, as described in example 6 of the cited Patent, leads to xcex1,xcex1xe2x80x2-bis(hydroxymethyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid. 
In the following, some preparation examples according to the method of the present invention are reported.